Install Duct Work
IMPORTANT: Install duct work in accordance with NFPA 90B
and any local codes.
When the furnace is installed so that the supply ducts carry
air circulated by the furnace to areas outside the space
containing the furnace, the return air shall be handled by a
duct or ducts sealed to the furnace casing and terminated
outsides the space containing the furnace.
If there is no complete return air duct system, the return air
connection must be sealed to the furnace casing and run full
size to a location outside the utility room or space housing
the furnace to prevent a negative pressure on the venting
Installation with Return Ducts
A return air duct system is recommended. If the unit is installed in
a confined space or closet, a return connection must be run, full
size, to a location outside the closet. The air duct in the closet
must be tight to prevent any entrance of air from the closet into
the circulating air.
Installation with an Evaporator Coil
IMPORTANT: When an air conditioning unit is used in
conjunction with the furnace, the evaporator coil must be
installed in the discharge (supply) air. Do not install an evaporator
coil in the return air; excessive condensation will occur within the
When installing a WEC coil in a horizontal position with a
horizontal gas furnace, the open end of the A-coil must face the
supply air outlet of the furnace (blow into the open end of the A-
coil). The A-coil should point away from the supply air outlet of
the furnace as shown.
Installation without an Evaporator Coil
If a cooling coil is not installed with the furnace, then a removable
access panel should be provided in the supply plenum for
purposes of inspecting the heat exchanger. This opening must be
accessible when the furnace is installed. It must be large enough
that the heat exchanger can be viewed for possible openings
using light assistance or so that a probe can be inserted for
sampling the airstream. The cover for the opening must be leak
Filters are not supplied with these furnaces; however, filters must
be used. It is the installer's responsibility to install a filter rack with
the duct work and to install properly sized filters in accordance
with the "Minimum Filter Requirements Chart."
The Airflow Descriptor is the 2 digits immediately preceding
the hyphen (-) in the furnace model number. Example: 16 is
the Airflow Descriptor for furnace model WFAT075B16-1A.
The model number is located on the rating plate inside the
Areas and dimensions shown for cleanable filters are based
on filters rated at 600 ft per minute face velocity.
Typical filter sizes are shown; however, any combination of
filters whose area equals or exceeds the minimum area
shown is satisfactory.
If a central return air filter-grille is used, the furnace does not
require a filter.
Minimum Filter Requirements Chart
Qty. Min. Area
16 x 20
20 x 20
20 x 25
16 x 20
20 x 20
20 x 25